1. Field of the Invention
The present invention relates to improved polyamides. More particularly, the present invention relates to inherently light- and heat-stabilized polyamides. The present invention further relates to a process for preparing these polyamides and to their use.
2. Description of the Related Art
The heat stability of polyamides, including nylon 6 and nylon 66, is insufficient for some applications. For instance, coloration problems can arise as a result of chemical changes (oxidative/thermal damage) to the polymer during carpet yarn or textile fabric heat setting. Both continuous filaments and staple fibers may be affected. It is known to add stabilizers to the polyamide to improve these properties. Such an addition can take place before, during or after the polymerization, for example during the processing. The customary known stabilizers are mixed into the polymer and are not bonded to the polymer chain. During processing or use they can migrate, evaporate or wash out of the polymer more or less readily, so that the effectiveness of the stabilization decreases in an undesirable manner and the surroundings (air, dyebath) may become contaminated.
DE-A-20 40 975, Sankyo Co. Ltd., describes the stabilization of synthetic polymers, including polyamides, with 4-aminopiperidine derivatives. Among the multiplicity of 4-aminopiperidine derivatives disclosed therein is 4-amino-2,2,6,6-tetramethylpiperidine (ef. No. 32 on page 8 of the document). However, this piperidine derivative is neither particularly singled out nor used in any Example. According to this reference, the 4-aminopiperidine derivatives are mixed with the ready-prepared polymer without becoming attached to the polymer chain.
DE-C-39 32 912, Sandoz, concerns polyamides containing radicals with sterically hindered amino groups, especially 2,2,6,6-tetramethyl-4-piperidyl radicals, incorporated in the molecule. The number of radicals is from 5 to 200 per polyamide molecule on average. According to this reference, these polyamides are useful, inter alia, for improving the dyeability of polyamides and as light stabilizers for plastics; they are to be incorporated in amounts of 1-10% by weight, particularly in the melt.
A paper in Poly. Deg. and Stab. 21, 251-262 (1988), states that the light stability of nylon 66 is improved on addition of 2,2,6,6-tetramethyl-4-piperidinol (TMP). The authors assume (see p. 259) that the TMP has reacted with the carboxyl end groups of the polyamide during a melt postcondensation of the TMP-including nylon 66 at 275xc2x0 C. under a water vapor atmosphere. But, they say, there are signs of (undesirable) crosslinking during the later stages of irradiation.
It is known to use amines or mono- and dicarboxylic acids as chain regulators in the polymerization of polyamides, and monocarboxylic acids are very predominantly used for this purpose in practice.
It is an object of the present invention to provide inherently light- and heat-stabilized polyamides and processes for preparing them.
We have found that this object is achieved when a triacetonediamine compound of the formula 
where R is hydrogen (4-amino-2,2,6,6-tetramethylpiperidine) or hydrocarbyl having from 1 to 20 carbon atoms, preferably alkyl (4-amino-1-alkyl-2,2,6,6-tetramethylpiperidine) having from 1 to 18 carbon atoms, or benzyl, is added before or in the course of the polymerization of the polyamides.
The present invention accordingly provides a process for preparing polyamides, which comprises effecting the polymerization of starting monomers in the presence of at least one triacetonediamine compound of the formula 
where R is hydrogen or hydrocarbyl having from 1 to 20 carbon atoms, preferably alkyl having from 1 to 18 carbon atoms, or benzyl. Preferred embodiments of the process of this invention are described in subclaims. The present invention further provides an inherently light- and heat-stabilized polyamide containing an amine radical of the formula 
where R is as defined above, chemically bonded to the polymer chain. Preferred polyamides of this invention are defined in corresponding subclaims.
The triacetonediamine compound is added to the starting monomers or the polymerizing reaction mixture and becomes bonded to the end of the polymer chain through reaction of its primary amino group with the starting monomers or with the carboxyl groups of the polyamide being formed. The secondary amino group of the triacetonediamine compound does not react because of steric hindrance. Thus, the triacetonediamine compound also acts as a chain regulator.
The chemical bonding of the triacetonediamine compound to the polymer chain of the polyamide results in inherently stabilized polyamides being obtained. The process of this invention thus offers the advantage of obviating the otherwise necessary separate step of mixing a stabilizer into the polyamide. This eliminates problems or quality reductions as can arise on incorporation of a stabilizer following surface application to the polyamide granules as a result of incompatibility, viscosity degradation, migration, vaporization or washoff of the stabilizer or a twofold stress as with compounding, for example. The use of the triacetonediamine compound in the process of this invention protects the polyamides against damage by the action of heat and thermal oxidation in processing and use.
The polymerization of the starting monomers in the presence of the triacetonediamine compound is preferably carried out according to customary processes. For instance, the polymerization of caprolactam in the presence of triacetonediamine (R=H) can be carried out for example according to the continuous processes described in DE 14 95 198 and DE 25 58 480. The polymerization of 66 salt in the presence of triacetonediamine can be carried out by the customary batchwise process (see: Polymerization Processes p. 424-467, especially p. 444-446, Interscience, New York, 1977) or by a continuous process, for example as described in EP 129 196. In principle, the triacetonediamine compound and the starting monomers can be introduced into the reactor separately or as a mixture. The triacetonediamine compound is preferably added according to a predetermined amount/time program.
In a preferred embodiment of the process of this invention, the starting monomers used for polymerization are caprolactam or at least one dicarboxylic acid A selected from adipic acid, sebacic acid and terephthalic acid and at least one diamine selected from hexamethylenediamine and tetra-methylene-diamine, or dicarboxylic acid-diamine salts thereof. Caprolactam is particularly preferred. Dicarboxylic acid A is particularly preferably adipic acid or terephthalic acid. Given the appropriate choice of starting monomers, the polymerization will lead to the preferred polyamides nylon 6, nylon 66, nylon 46 or nylon 610.
In a preferred embodiment, the triacetonediamine compound is added to the starting monomers in an amount of from 0.03 to 0.8 mol %, preferably from 0.06 to 0.4 mol %, based on 1 mol of carboxamide groups of the polyamide. This statement of quantity relates for example to 1 mole of caprolactam when nylon 6 is to be prepared or to 0.5 mol of 66 salt when nylon 66 is to be prepared. It was found that amounts below 0.03 mol % do not ensure sufficient stabilization, whereas amounts above 0.8 mol % make it impossible to achieve the desired degree of polymerization owing to the regulating effect of the triacetonediamine compound.
In a preferred embodiment of this invention, the triacetonediamine compound is combined with at least one customary chain regulator. Examples of suitable chain regulators are monocarboxylic acids such as acetic acid, propionic acid and benzoic acid. The chain regulator combination and the amounts used are selected inter alia according to the amino end group content desired for the end product and according to the desired melt stability. The amino end group content depends on the dyeability desired for the filaments or fibers. The melt stability depends on the practical requirements of processing products, especially in relation to spinning.
The nylon 6 (polycaprolactam) prepared by the process of this invention, as well as the triacetonediamine compound, preferably contains dicarboxylic acids B as chain regulators. More particularly, such nylon 6 products, as well as possessing the requisite melt stability, the desired filament or fiber dyeability and good light and heat stability, also possess improved strength for the filaments obtained, especially filaments produced by high-speed spinning at very high rates of speed.
The dicarboxylic acids B used as chain regulators in combination with the triacetonediamine compound can be identical to or different from the dicarboxylic acids used as dicarboxylic acid A. They are preferably selected from: C4-C10-alkanedicarboxylic acids, especially adipic acid, azelaic acid, sebacic acid and dodecanedioic acid; C5-C8-cycloalkanedicarboxylic acids, especially cyclo-hexane-1,4-dicarboxylic acid; and also benzene- and naphthalene-dicarboxylic acids, especially isophthalic acid, terephthalic acid and naphthalene-2,6-dicarboxylic acid. The dicarboxylic acids B are preferably used in an amount of from 0.06 to 0.6 mol %, preferably from 0.1 to 0.5 mol %, based on 1 mol of carboxamide groups of the polyamide.
In another preferred embodiment, the polymerization of the process of this invention is carried out in the presence of at least one pigment. Preferred pigments are titanium dioxide or inorganic or organic coloring compounds. The pigments are preferably added in an amount of from 0 to 5 parts by weight, especially from 0.02 to 2 parts by weight, based on 100 parts by weight of polyamide. The pigments can be added to the reactor with the starting materials or separately therefrom. The use of the triacetonediamine compound (also as chain regulator constituent) distinctly improves the light stability of the polymer compared with a polymer comprising only pigment and containing no triacetonediamine.
This invention also relates to the use of inherently light- and heat-stabilized polyamide of this invention for producing filaments, fibers or films. This invention further relates to a process for producing filaments based on polycaprolactam by high-speed spinning at takeoff speeds of at least 4000 m/min and to the filaments thus obtained. In addition, this invention encompasses the use of filaments obtained according to this invention for producing fibers and fabrics and also the fibers and fabrics obtainable by this use.
The Examples which follow illustrate the invention.
General remarks concerning the Examples
The relative viscosity of the polyamides (pellets and filaments) was determined in 1% strength solution (1 g/100 ml) in concentrated sulfuric acid (96% by weight) at 25xc2x0 C. The end group content was determined by acidimetric titration. The amino end groups were titrated with perchloroacetic acid in a solution in 70:30 (parts by weight) phenol/methanol. The carboxyl end groups were titrated with potassium hydroxide solution in a solution in benzyl alcohol.
The level in the polyamides of the triacetonediamine compound and of any dicarboxylic acids can be determined by hydrolyzing a sample in dilute mineral acid and analyzing the hydrolyzate by customary methods, for example by gas chromatography.
The heat stability of the polyamide filaments was determined under conditions which substantially correspond to those of heat setting processes in subsequent treatment stages, for example heat setting of BCF (bulked continuous filament) or tenter setting of textile fabrics. 5 g hanks of the drawn filaments were rapidly introduced on a holder together with the comparative samples into a through-circulation oven preheated to 185xc2x0 C. and left therein for 120 seconds from reattainment of the air temperature measured in direct sample vicinity. The sample was then immediately removed and cooled down in air at 20xc2x0 C. room temperature. Filaments to be compared were treated together.
The damage incurred (compared with an untreated sample of the same filament) was determined through the decrease in the relative viscosity and the amino group content and the increase in the carboxyl group content.
The absolute decrease in the basic groups is then converted into a percentage decrease, based on the untreated yarn sample, to arrive at a more useful figure for actual service.
The ultimate extension was determined using an Uster Tensorapid I and a clamped length of 200 mm in the case of partially oriented yarn (POY) filaments, of 500 mm in the case of drawn and textured filaments. The filament time-to-rupture was within the range 20xc2x12 seconds. The pretensioning force was 0.025 cN/dtex in the case of POY and 0.05 cN/dtex in the case of drawn filaments.
The tenacity RH was calculated according to the following equation:
RH=FH/TtV 
where FH is the ultimate tensile strength [cN] and TtV is the original linear density [dtex]. The ultimate tensile strength value used was the largest value obtained in the ultimate extension measurements.
The ultimate extension EH was determined as the ratio of the length change xcex94l at the moment of attainment of the ultimate tensile strength to the original length lV of the sample according to the following equation:
EH=xcex94lxc2x7100%/lV 
where xcex94l is the difference in the length of the sample at the time of application of the ultimate tensile strength, lH, and the original length lV.